1. Field of the Invention
The present invention relates to a high transmittance glass sheet of soda-lime-silica glass manufactured mainly by a float process. More specifically, this invention relates to a high transmittance glass sheet that allows the formation of nickel sulfide (NiS) in a process of melting a glass raw material to be suppressed effectively.
2. Related Background Art
In methods of manufacturing a soda-lime-silica glass sheet such as a float process and a roll out process, the following problem may arise. That is, in a process of melting raw materials in a furnace at a high temperature near 1,500° C., metal particles of stainless steel or the like containing nickel (Ni) may be mixed into the raw materials. The metal particles may react with sulfur (S) in salt cake (Na2SO4) included in the raw materials. As a result of this reaction, nickel sulfide (NiS) is formed as minute foreign matter in glass products. NiS particles are present at a minimal rate of about one per a little over 10 tons of glass products, and are of an extremely minute spherical form having a diameter as small as about 0.3 mm or less. Therefore, it is difficult to detect NiS particles on production lines.
Some soda-lime-silica glass sheets are tempered to be used for buildings, vehicles, cover glass plates for solar cell panels, solar water heaters or the like. In a tempering process, a glass sheet is heated to a temperature near the softening point (about 600° C.) of the glass sheet. Then, the glass sheet is quenched so that compressive stress layers are generated in surfaces of the glass sheet.
When NiS is contained in a tempered glass sheet, NiS is present in an α phase that is stable at about 350° C. or higher, and undergoes phase transition with the lapse of time to a β phase that is more stable at room temperature. This phase transition causes NiS particles to expand in volume. As a result of this, micro cracks may appear in the vicinity of the NiS particles. Inside the tempered glass sheet, a tensile stress layer exists, having a thickness of about two-thirds that of the glass sheet. When cracks appear in the tensile stress layer, the cracks run rapidly to cause spontaneous fracture of the tempered glass sheet.
To prevent such spontaneous fracture of a tempered glass sheet, so-called soaking has been known. In this method, tempered glass sheets are heated to 300° C. or lower in a furnace (soaking furnace). Then, the tempered glass sheets are maintained in the furnace for a predetermined time, so that NiS undergoes phase transition from an α phase to a β phase. This forces the tempered glass into breakage. In this manner, defective glass products containing NiS are eliminated.
However, operations such as the soaking in which heat treatment is mainly performed cost considerable energy and time, thereby causing an increase in manufacturing cost. This also is a serious hindrance to shortening of delivery times and an improvement in productivity. Further, defective products are eliminated in the soaking, thereby causing a decrease in product yield.
JP 9(1997)-169537 A discloses a method of manufacturing a soda-lime-silica glass in which 0.01 to 0.15 wt. % of a zinc compound such as zinc nitrate and zinc sulfate is added to raw materials, thereby allowing the formation of NiS to be suppressed.
Meanwhile, there has been a growing demand that a high transmittance glass sheet, more specifically, a glass sheet having a light color and a high transmittance be used for an interior glass, a showcase, a display case, a high transmittance non-colored window glass, a high transmittance non-colored mirror, a glass substrate for a solar cell panel, a cover glass plate for a solar cell panel, a solar water heater, a material for a high solar-heat transmittance window glass, and a flat display substrate glass such as a front panel or the like. However, no high transmittance glass sheet has been known so far that is suitable for industrial mass production.